Bumped to 1.6 in preparation for next release.
Compatibility improvements for GNOME Keyring backend

- Use the same attributes (``username`` / ``service``) as the SecretService
  backend uses, allow searching for old ones for compatibility.
- Also set ``application`` attribute.
- Correctly handle all types of errors, not only ``CANCELLED`` and ``NO_MATCH``.
Secret Service: use a better label for passwords, the same as GNOME Keyring backend uses
GnomeKeyring: Avoid printing warnings to stderr when GnomeKeyring is not available